This invention relates to optical clock recovery from a line signal containing both data and clock information.
The basic elements of any digital line transmission system consist of a transmitter and a receiver connected by a transmission medium. An example of such a system is the optical fibre digital line transmission system where the source transmitter is a laser, the receiver is a photodetector system and the transmission medium is the optical fibre. The digital line transmission signal must contain both data and clock information so that these signals may be properly reconstituted at the receiver. A method of achieving this is to encode the data signal, typically an NRZ signal, prior to transmission. An example of this is the 5B6B coding system often used for optical transmission or the CMI coding system often used for electrical transmission. This process ensures that the line transmission signal contains a clock component which can be recovered at the receiver where decoding also takes place to recover the data. Typically in prior art digital line transmission systems the receiver terminates the transmission line with a photodetector which provides a digital electrical representation of the line transmission signal. The transmission clock is typically extracted from this electrical signal, using a phase-locked loop (PLL) to synchronise the output of a local oscillator with the clock content of that signal to enhance the efficiency of this clock signal produced by the receiver's detector extraction process, the electric signal produced by the receiver's detector may be modified by differentiation and square law filtering to provide a modified signal with an enhanced power component at the clock frequency. A circuit to achieve this may contain many discrete components or be comprised of a custom device. In both instances the circuit is relatively expensive first to develop and then to produce. For use at high frequency bit rates the manufacture of large volumes of PLL-based clock extraction circuits can be costly in terms of setup required at the final stages of product test. Additionally each circuit has to be specifically designed for operation at a specific line transmission rate, with the result, for instance, that a clock extraction circuit for a 678 Mbit/s optical transmission system can not be used or adjusted to work in a 140 Mbit/s line transmission system. There are alternatives to the use of a local oscillator and phase-locked loop for electrical clock extraction, thus the electrical signal can be amplified and filtered using a high Q SAW filter, but these filters similarly have the disadvantage of being bit-rate specific.
As an alternative to all-electronic clock recovery from a detected optical clock signal, recovery can instead be achieved optically or opto-electronically using a self-pulsating laser diode (SPLD) or non-linear optical amplifier (NLOA) comprising an injection laser, diode typically a buried heterostructure laser diode, with a top contact divided into two parts which are independently powered respectively to provide a gain region and a self-absorption region optically in tandem. Thus P. E. Barnsley et. al., in a paper entitled "Clock Extraction Using Saturable Absorption in a Semiconductor Nonlinear Optical Amplifier" (IEEE Photonics Technology Letters Vol. 3, No. 9, Sept. 1991, pages 832-4), describe the use of one of these devices to recover an electrical clock signal from its saturable absorber contact when the optical signal is applied to its input facet. The paper explains, with particular reference to its FIG. 2, that the signal obtained directly from this contact requires to be filtered before it is usable as a clock signal, and hence this form of clock recovery, like the ones mentioned earlier, suffers from the problem that any particular implementation is bit-rate specific.
In two other papers by P. E. Barnsley et. al., respectively entitled "All-optical Clock Recovery from 5Gb/s RZ Data using a Self-Pulsating 1.56 um Laser Diode" (IEEE. Photonics Technology Letters Vol. 3, No. 10, Oct. 1991, pages 942-5) and "A 4.times.5 Gb/s Transmission System with All-Optical Clock Recovery" (Ibid. Vol. 4, No. 1, Jan. 1992, pages 83-86) there is described a method of clock recovery which is all-optical, and in which the need for electronic filtering is avoided because the filtering is performed optically by the action of the SPLD device itself. However a disadvantage of this approach is that clock recovery involves the taking of a significant proportion, typically 20%, of the optical power which would otherwise have been available for signal detection.